Lesbians are more than twice as likely as heterosexual women to have a hormonal imbalance that causes infertility, a British study has found, linking for the first time lesbian sexual orientation with hormonal makeup.

The discovery by researchers from the London Women's Clinic centred on the most common cause of ovarian dysfunction in women, polycystic ovarian syndrome (PCOS).

Caused by an imbalance of sex hormones, one of the main features of PCOS is hyperandrogenism - an abnormally high concentration of male steroid hormones called androgens. Symptoms related to PCOS include irregular menstrual periods, an inability to conceive, acne or excessive body or facial hair.

The study involved examining 618 women who attended the clinic for fertility treatment between November 2001 and January 2003. Each woman had her medical and sexual histories taken during three sessions with a nurse, a counsellor or a clinical psychologist. Of those interviewed, 254 were lesbian and 364 were heterosexual women.

None of the women studied knew if they had polycystic ovaries seen in PCOS, but 15% had been treated previously for some of the symptoms. The subjects were given a pelvic ultrasound examination on the second or third day of their menstrual cycle, and blood samples taken to measure their levels of reproductive hormones.

The incidence of both polycystic ovaries and PCOS is generally twice as high in women seeking fertility treatment than it is in the general population. In the study, 52% of the women had polycystic ovaries, and 24% had PCOS. But the study showed prevalence of polycystic ovaries and PCOS was much higher in lesbian women than heterosexual women.

Eighty percent of the lesbian women had polycystic ovaries, compared to only 32% in heterosexual women. And lesbian women had higher rates of PCOS as well, with 38% having the syndrome compared to 14% of heterosexual women.

"We observed a significantly higher prevalence of polycystic ovaries and PCOS in lesbian compared with heterosexual women. Our initial results are also suggestive of a significantly greater hyperandrogenism in lesbians compared with heterosexual women," said Agrawal.

Lesbian and heterosexual women with normal ovaries had similar levels of androgen and SHBG (sex hormone binding globulin), which binds testosterone and other sex hormones.

SHBG, on the other hand, was significantly lower in lesbian women. If SHBG levels are lowered, a woman with normal levels of testosterone will have elevated levels of free testosterone - which results in symptoms related to hyperandrogenism.

"Our research neither suggests nor indicates that polycystic ovaries-PCOS causes lesbianism, only that polycystic ovaries-PCOS is more prevalent in lesbian women," said Agrawal. "We do, however, hypothesise that hyperandrogenism - which is associated with PCOS - may be one of the factors contributing to the sexual orientation of women."

The researchers also pointed out that in the past 20 years, only 0.1% of published research was dedicated to the healthcare of homosexual individuals. "Our study emphasises the importance of treating these women in a non-judgemental and non-biased manner, so that clinicians may offer them appropriate health advice," Agrawal said.